Incandescent lamps and fluorescent lamps have been used as recent mainstream illumination systems. The incandescent lamps utilize light emission obtained by current heating of tungsten filaments. The fluorescent lamps are a type of low-pressure discharge lamps enclosing mercury vapor, and utilize emission of visible light by phosphors due to an ultraviolet radiation spectrum of mercury excited by electrons.
An incandescent lamp has a low light-emission efficiency, and discharges a large amount of heat. For example, a gas-filled 100 W single-coil tungsten bulb has a light-emission efficiency of 14 lumens per watt (lm/W). An incandescent lamp can provide various types of projection illumination as a point light source, but generates a large amount of heat.
In contrast, a fluorescent lamp has a high light-emission efficiency, and can be a nearly ideal light source. For example, a general 20 W white fluorescent lamp has a light-emission efficiency of 40 to 60 lm/W and a high color-rendering index of 90 or more. In particular, a cold cathode fluorescent tube achieves a high luminance of 10,000 cd/m2 or more in a narrow tube, and is widely used as a backlight for an LCD.
The fluorescent lamp, however, basically has a tubular shape because of the structure thereof, and thus, is disadvantageously limited in shape. In addition, because of the presence of mercury, the fluorescent lamp is not preferable in consideration of environmental issues.
For the reasons described above, the current mainstream illumination systems have disadvantages, and are not perfect. As other illumination systems, arc lamps and high-pressure mercury-vapor lamp are also used. These lamps including incandescent lamps and fluorescent lamps, however, are limited to the tube or bulb shapes, and are limited in size. In addition, most of these lamps are used for emitting white light.
Under these circumstances, demand for thin illumination systems has increased in recent years.
In view of this, the use of inorganic EL devices is expected. However, the inorganic EL devices have problems of requirement of high voltages, low light-emission efficiencies, and limited colors of emission, and actually have low possibilities of practical use.
On the other hand, organic EL devices have their performance significantly improved with research and development for application to displays, and are more and more expected to be practically used for illumination systems. The organic EL devices have advantages of easy dimming and being free from mercury.
At present, however, the organic EL devices are inferior to fluorescent lamps in terms of light-emission efficiency, color rendering properties, luminance, lifetime, and cost, for example. The light-emission efficiencies of the organic EL devices have been rapidly increased by active developments such as reduction of voltage with chemical doping techniques, increase in light-emission efficiency with the use of phosphorescent materials, and increase in lifetime with introduction of tandem constructions. However, in terms of cost, even in the case of application to illumination systems, since fabrication methods for such illumination systems are merely an extension of fabrication methods for displays, the cost thereof is greatly different from that for general illumination systems.
An organic EL device is fabricated through a large number of complicated processes such as stacking of a positive electrode, an organic layer, and a negative electrode, for example, patterned into predetermined shapes on a substrate. These processes tend to increase the fabrication cost. In addition, a large glass substrate is often employed as the substrate. This glass substrate is difficult to handle, and has a problem of a large initial investment.
As a solution to the problems described above, a roll-to-roll process is proposed (see Patent Document 1). In the roll-to-roll process, a film is deposited while a film substrate is wound from a roll to another roll, thereby significantly enhancing productivity.
However, in terms of performance, an organic EL device has problems, such as variation in light emission, degradation due to heating, and increase in power consumption, caused by a voltage drop.
Specifically, in the case of a current-driven method, as the distance from a power source to a light-emission point increases, the voltage decreases due to wiring resistance. Consequently, the applied voltage varies depending on the location of the light-emission point, causing a variation in light-emission, for example.
In particular, as the size of the organic EL device increases, the influence of a voltage drop increases. Thus, the solution to this problem is important. In addition, in the organic EL device, a conductive film transparent to visible light is often employed as an electrode for light extraction. This conductive film, however, has an electric resistance higher than that of a metal film, and thus, is easily affected especially by a voltage drop.
To reduce the influence of the voltage drop, a light-emission apparatus in which an auxiliary wire of a metal film is electrically connected to a conductive film in order to reduce the wiring resistance of a conductive film is proposed (see Patent Document 2).